Formation of a molded plastic article typically involves a number of operations, such as introducing plastic material (e.g., molten plastic material) into or onto a mold, forming the molded article within or on the mold (which usually involves a cooling sequence), and removing the molded article from the mold. In the case of compression molding, a compression operation is undertaken in which, for example, an upper mold half is compressed against plastic material previously introduced into or onto a lower mold half.
While the various molding operations may be conducted at the same location, this can result in production inefficiencies. For example, during the molding, cooling and de-molding operations, the apparatus used to introduce plastic material into the mold (e.g., an extruder) typically must rest idle, until the other operations are completed and the mold is cleared.
To improve production efficiencies, the various molding operations may be positioned at separate locations or stations, and one or more (typically two or more) molds are moved between the various stations. This allows for the contemporaneous performance of different operations. In one approach, two or more molds are moved back and forth along a linear line having separate mold operation stations (e.g., separate polymer injection, molding or compression, and de-molding stations). With a linear line approach, however, usually at least one operation must be duplicated, typically at or near the terminal ends of the linear line, which can result in increased equipment costs. For example, a de-molding station may have to be located at both ends of the linear line, with a separate compression molding station interposed between each terminal de-molding station and a single centrally located polymer introduction station.
In another approach, a rotary arm system having a mold at the end of each arm may be employed. With each registered turn of the rotary arm system, each mold is moved from one station to the next. Rotary arm molding systems are not, however, particularly well suited for use with molding processes that require one or more molds to be moved at different linear velocities. With rotary arm molding systems, which are typically rigid, as one arm moves, so do all the other arms. For example, in a compression molding process, the lower mold may have to be moved reciprocally and/or at variable linear velocities while plastic material is introduced therein (e.g., to achieve a substantially even distribution of plastic material over the interior mold surface). Linear movement of the molds during the other operations, such as the compression molding operation, is typically undesirable. In addition, the formation of large molded articles requires the construction of a large, costly and often heavy rotary arm system, which can be difficult to operate.
There is a need then to develop new methods and systems for forming molded plastic articles. In particular, it would be desirable that such newly developed methods and systems allow for the contemporaneous performance of separate and preferably different molding related operations, coupled with a minimum of operational and equipment duplication, and improved production efficiencies.